Another West Nile Virus Summer?

West Nile Lessons from 2003 continued...

"Nobody really understands the flattening out. We've seen it with St. Louis encephalitis virus in years before," Campbell says. "Part of it is bird immunity."

Most researchers think that people once infected with West Nile virus get very long-lasting immunity. That happens with similar viruses, such as the yellow fever virus. For every person who comes down with West Nile symptoms, four more never have any noticeable symptoms at all. But Campbell says that even in intensely populated areas with lots of infected birds and mosquitoes, no more than 4% of people show signs of having been infected with the virus. An unreleased study of people in Slidell, La., Campbell says, found that West Nile-infected mosquitoes bit only about 2% of the population.

Hot Spots: California, Colorado

This pattern bodes ill for California. Already this year, a dead crow and two live house finches in Southern California tested positive for West Nile virus.

"We worry about Southern California. By that theory, California should light up this year -- but it all depends on climate and mosquito control and luck," Campbell says. "If California is going to be a problem, you might see human cases very early. Cases in May or June or early July are very unusual and could be a sign of something big. Just a lot of dying birds and horses could suggest something big coming."

The pattern also suggests that Colorado -- last year's hardest hit state -- may see fewer cases this year.

"The big question is, what will happen in Colorado this summer," Campbell says. "If we continue to have a hot, dry summer, we could have activity again this year. If you start seeing cases in early July rather than late July in Colorado, it might portend a big epidemic."

It's too soon to tell whether this year's first probable human case of West Nile virus -- seen in a 79-year-old man from southern Ohio with severe viral encephalitis -- is an unusual aberration or a sign of things to come. Ohio reported 108 cases of West Nile virus infection in 2003 -- down from 441 cases in 2002.